Irradiation of SiO2 with soft X-ray photons (hv > 100 eV) produces a variety of defects, of which E-l' centers and neutral Si-Si bonds are mainly responsible for the dielectric response change. The thermal processes that modify the structures around the defect sites have been investigated by in situ spectroscopic ellipsometry. Annealing the irradiated SiO2 film diminishes the number of defects which are assigned to E-l' centers by about half. The competing channels for annihilation of E-l' centers are the recovery of the Si-O-Si bonding configuration and, in the opposite direction, the decomposition of the material into volatile products until the network is completely restructured. The other half of the defects are converted to Si-Si bond units and precipitates as nanocrystalline particles of Si. (C) 2002 Elsevier Science B.V. All rights reserved.